1. Field of the Invention
This invention relates to a system for towing equipment at sea, especially in seismic surveys, comprising a main vessel adapted to tow the equipment in a chosen direction of movement, at least one deflector on each side of the main vessel adapted to, during the towing, pull at least part of the towed equipment transversely in relation to the direction of movement.
2. Description of the Prior Art
In seismic studies at sea acoustic sources are used being mounted in or close to a vessel, and directing acoustic signals toward the sea floor. Reflections of these signals are received by a number of receivers being positioned in seismic cables being towed behind the vessel. These cables are spread out to a desired width to provide measurements of the geological conditions over an area with a certain width. As the use of the seismic vessel is expensive, it is advantageous to make the width of the tow as large as possible, with a large number of seismic cables, so that one signal pass over the area to be measured covers as large an area as possible. To spread the seismic cables transversely in relation to the seismic vessel deflectors are usually used.
Such deflectors are traditionally passive devices comprising one or more wings providing a lift in the required direction. Because of the towing resistance in the water, caused both by the deflector and the towed cables, there are, however, limits to the lift which may be obtained using passive deflectors, which in turn, together with a wish in seismic studies to keep the seismic cables as close to the vessel as possible, limits how far the deflectors may be pulled transversely. When the deflector is used in seismic surveys it will in addition be loaded with the seismic cable to be pulled sideways. Today there is a limit to the width of the cable tow with passive deflectors being approximately 800 meters, with approximately 10 seismic cables.
Many types of active systems have been suggested in different occasions to obtain a lift, e.g., using the so-called Magnus effect. This means that a rotating cylinder being moved in a medium in a direction perpendicular to its axis will experience a lift. The lift pulls in the direction of movement of the part of the cylinder facing the flow. This effect has been suggested in many applications, such as shown in U.S. Pat. Re 18,122 (Flettner). A disadvantage related to active deflectors is that they are dependent on power supplies. If the power supply fails, the lift is lost. In seismic surveys, a power failure makes it impossible to keep the seismic cables separated, with resulting damages to equipment and cables. Usually the active systems require too much power to allow the power supply to be kept in the deflector. Therefore, the power has to be transmitted from the towing vessel using a cable to the deflector. This cable will in turn increase the towing resistance and thus the power consumption, and also make the system vulnerable to failure in the power supply.
Similar problems also occur if there is a need for different active control systems on board the deflector, for example for adjusting the position of the deflector, the angle in relation to the moving direction and possibly the lifting ability.